The present invention relates to an image transfer apparatus and a method, and more particularly to a method for preparing a color proof by way of image transfer in a reproduction process.
For producing a number of prints with a printing plate constructed of a reproducing material, for example, a PS (presensitized) plate, from a color original, it is customary to generate halftone negatives or halftone positives in four colors from the color original with a color scanner, then patch up, reverse, and retouch the halftone negatives or halftone positives to produce a single four-color positive, make a proof for correction or examination, and thereafter produce final prints. In the process of preparing the halftone negatives or positives, it is necessary to check color separation in the color scanner. The process of making the single positive requires checking of the patching-up and other steps. These checking steps are required for a better printing finish. Also, in the process of making the proof, the order of the prints has to be confirmed.
To meet the above checking requirements, it has been the conventional practice to prepare a color proof. The color proof is made of a photosensitive material used for a silver-salt photographic system or an electrophotographic system, or a photosensitive material of photopolymer. Color images have been formed by using pigments or dyes. The color proof can be prepared by either a negative-positive system or a positive-positive system.
An example of making such a color proof will be described. A photosensitive material composed of a support and a photosensitive layer disposed thereon is pressed against a transfer sheet of paper with heat to transfer the photosensitive layer to the transfer sheet, and then an image of a first color is formed on the transfer sheet by exposure and development. Likewise, a second photosensitive layer is transferred to the transfer sheet on the first-color image, and an image of a second color is thereafter formed thereon by exposure and development. The above process is repeated by exposure and development. The above process is repeated four times for yellow, magenta, cyan, and black to produce a multicolor image.
The photosensitive layer can be transferred to the transfer sheet in various ways, such as by pressing the layer against the transfer sheet with or without heat. As an example, one or both of a pair of rollers are employed as heated rollers, and the photosensitive material with the photosensitive layer and the transfer sheet are passed in superposed relation between the heated rollers for transferring the photosensitive layer to the transfer sheet.
According to the present invention, a photosensitive material is exposed to an image, and a developed image is transferred to an image-receiving sheet to produce a color proof. A multicolor image can be formed by preparing color images of yellow, magenta, cyan, and black, and then transferring these color images successively to one image-receiving sheet. With this method, since the formed images are successively transferred color by color, a higher accuracy for image transfer is required than would be the case in which images are formed by exposing photosensitive layers transferred to a transfer sheet. More specifically, since the ability for the images to be transferred to the image-receiving sheet is largely dependent on the transfer temperature, the temperature at which the images are to be transferred should be strictly controlled. Where a general transfer apparatus is employed, it has heretofore been necessary to employ a considerably high transfer temperature as the heating time is short. Therefore, the sheet inserted between the sheets is subjected to local elongation, causing misregistration of colors in the transferred images and transfer irregularities.
The photosensitive material or image sheet and the image-receiving sheet are formed of a soft material. When an image is transferred or the sheets are pressed by the heated rollers, the superposed sheets tend to be warped, positionally displaced from each other, or wrinkled, resulting in a reduction in the sharpness of the transferred image. Especially when producing a color proof, because normally four image transfer cycles are necessary, misregistration of colors is a serious problem.
Moreover, inasmuch as the temperature is required to be elevated to a considerably high level, the heater employed has to have a large thermal capacity, and accordingly the entire device is large in size and has a large electric power consumption. When the superposed sheets are inserted between the heated rollers, the heated rollers are subjected to a localized temperature reduction, thereby causing a color shift in the transferred image. Dependent on the size of a sheet to which an image is to be transferred, the heated rollers generally make several revolutions before the sheets pass through the transfer device. Therefore, the heated rollers have portions subjected to different surface temperatures as such portions contact the sheets in the first and following revolutions. As a result, the transfer temperature is lowered with time as the rollers rotate, thus causing transfer irregularities. Furthermore, since images are transferred normally from four image sheets to a single image-receiving sheet, the colors are liable to be shifted from one another.